Using Cisco for Remote Access

WAN connections consist of three types: circuit-switched such as modems and ISDN, dedicated or leased lines, and packet-switched such as ATM and Frame Relay. Learn how to choose the correct LAN type for your organization's needs.

This chapter is from the book

As technology increases in importance to businesses, wide area network (WAN)
connectivity becomes a primary factor in implementing business practices. WANs
are used to connect multiple sites together with a medium that can carry data
over longer distance than is possible with local area network (LAN) technology.
As businesses expand to new geographic regions, thereby becoming more
competitive in the marketplace, the types of connectivity through WANs can
differ greatly. In this chapter we will examine the different types of WANs that
can be used and the considerations a business (or consultant) must make to
effectively choose the right type.

Overview of WAN Connections

Although connecting all users to a LAN is ideal because of the high speed and
low cost, it is impractical to do so across large distances. These distances can
be as close as an office across the street or as far as an international branch
office halfway around the world. These distances require us to examine the types
of WANs that are available. For each type, we must keep in mind

Availability: Is the technology available in the area?

Bandwidth: How much do we need, and how much do we get for that type of
WAN connection?

Cost: Is there a cheaper connection type that still takes into account
future growth?

Ease of management: Is the initial configuration as well as normal
operation easy or difficult to maintain?

Quality of Service (QoS): How critical is the actual data itself, and is
there a way to ensure low or no data loss with this WAN type?

Security: What measures need to be in place to provide security of
company data, while still allowing users and customers to access the data they
need?

Reliability: Is the WAN link a critical link, and do we need an
additional link in the event of failure?

Application traffic: What is the primary type of data being sent across
the WAN, and can this WAN handle that type?

Connection Types

WAN connections can be broken down into three different types, depending on
how they carry data: dedicated, circuit-switched, and packet-switched. We will
examine each quickly and describe the different types of protocols that each
type can provide. We will examine many of these protocols in detail throughout
the rest of the book.

Dedicated Connections The first type of connection is a
dedicated connection that is used to provide full connectivity between two sites
in a point-to-point fashion. Also known as a leased line, this type of
connection is purchased from the telephone company (telco) and uses a permanent
path through the telco's infrastructure, from one site to another (Figure
1-1). There is no call setup and teardown, which means the circuit is always
available.

Since the company owns the line, it has full use of the bandwidth, whether it
is used or not. The speed of the link can range up to a T3, which is
approximately 45 Mbps. If the company is underutilizing that bandwidth, then the
cost of the dedicated line is high. The cost of the line can also be too great,
even if the bandwidth is being properly used, due to distance limitations. As
the distance increases and the possibility of crossing geographic (and telco)
boundaries appears, the price increases. Therefore, these lines tend to be best
used in short distances with a higher volume of traffic or a steady flow of
traffic.

This type of connection is usually done with a synchronous serial type of
connection. Cisco supports this type with virtually all of their routers, using
one or more different types of synchronous serial connections, including

EIA/TIA-232

V.35

X.21

EIA/TIA-449

X.21

EIA-530

HSSI

Circuit-Switched Connections There are two types of
circuit-switched connections available: asynchronous and ISDN (Integrated
Services Digital Network). In both cases the circuit, or dedicated path, is
created when the call is initiated to the remote site and the circuit is
destroyed when the call ends. The best example of a circuit-switched network is
the Public Switched Telephone Network (PSTN) that we use every day in our
lives.

Asynchronous circuits for data transfer are accomplished through a modem and
the use of the telephone network (Figure 1-2). Since telephones exist in
virtually every city in the world, connecting to remote sites is always a
possibility. While the cost of the telephone service is very cheap when compared
to other types of WAN connections, the real limiting factor is the small
bandwidth that is available. Depending on the setup of the connection, the best
that can be accomplished is 56 Kbps. Because of this, this type of connection is
best used when other WAN types are not available, small amounts of data are
exchanged, or cost is a primary issue.

ISDN has two flavors that are used for WAN connections. The first is Basic
Rate Interface (BRI) and has a maximum bandwidth of 128 Kbps. This is at least
twice as fast as a modem, and the call setup and teardown are much quicker. In
addition, having a BRI connection not only allows for data transfer, but analog
voice can be used at the same time to cut phone costs to remote sites. BRI tends
to be more expensive than asynchronous calls and has limited availability,
although ISDN is becoming more available throughout the U.S.

The second type of ISDN is known as Primary Rate Interface (PRI) and can
reach speeds up to 2 Mbps. This type of WAN connection is ideal for combining
multiple BRI channels and asynchronous calls into the same router, using only
one physical interface.

There is one other potential use for both asynchronous and BRI networks. In
the event that a primary link fails, they may act as a backup link to ensure
connectivity.

Packet-Switched Connections Packet-switching (Figure 1-3)
is a method where two or more sites are connected through a shared network,
typically called a cloud. By shared network, we mean that more than one company
has access to the cloud. Remote sites are connected via a virtual circuit (VC)
that allows data to traverse the cloud and arrive at the correct location.
Within the cloud, each packet can take a different path to reach the final
destination. Because the data travels through a shared cloud, the cost tends to
be lower than the same bandwidth used for a dedicated line.

Packet switching can be considered the common ground between dedicated lines
and circuit-switched. Although usually more expensive and not as freely
available as circuit-switched networks, the additional bandwidth (up to T1
speeds) makes it an attractive alternative. Also, it is cheaper over longer
distances than dedicated lines, which again makes it a nice alternative.

Protocols Used on WANs

There are many different types of protocols used throughout the world,
depending on location, type of WAN, and administrator knowledge. These protocols
all operate at layer 2 (at least) of the OSI model (data-link layer). We will
briefly review them here, and other chapters will dedicate more discussion to
the more common of them.

Point-to-Point Protocol (PPP) PPP is used with both
dedicated lines and circuit-switched networks. It is a standard protocol that
vendors can use to interoperate their equipment with other vendors. In addition,
PPP supports multiple network layer protocols such as TCP/IP and IPX/SPX, as
well as authentication and compression mechanisms. Because PPP is such a
versatile protocol, we will examine it more thoroughly in later chapters.

Serial Line Internet Protocol (SLIP) One of the earliest
protocols used in point-to-point connections, SLIP is being phased out due to
some of its serious drawbacks. These drawbacks include support only for TCP/IP
and lack of security. SLIP can be used over dedicated and circuit-switched
networks. We will not examine SLIP in this book.

High-Level Data Link Control (HDLC) Although HDLC is a
standard, the limiting factor of that standard is the support for only a single
protocol. Because of this, Cisco has modified it to support multiple protocols
over point-to-point links. This is the default encapsulation protocol on serial
links with Cisco routers. Because of this modification, though, Cisco products
may not interoperate with other vendors' equipment. In this situation, PPP
is the better protocol. HDLC is supported over dedicated lines, but not over
circuit-switched or packet-switched networks. HDLC is a simple protocol, and
therefore we will not examine it in much more detail in this book.

Frame Relay (FR) Frame Relay was a protocol derived from
ISDN specifications and has evolved to become the dominant layer 2 protocol over
packet-switched networks. As a standard, it has become widely available
throughout the United States and many other parts of the world. Frame Relay is a
protocol that should be thoroughly understood, and so we will examine it in
later chapters.

X.25 X.25 protocol is an older protocol that can still be
found throughout the world. It is also a protocol that is used over
packet-switched networks. The primary difference between X.25 and Frame Relay is
the overhead of error correction built into X.25. This overhead was a necessary
feature of the older, less reliable networks and networks still found in more
remote parts of the world. Although it is an older protocol, it does have a
large install base worldwide, and therefore we will examine it in more detail in
a later chapter.

Future Networks Asynchronous Transfer Mode (ATM) networks
are cell-switched instead of packet-switched. This smaller, fixed cell size of
53 bytes allows multiplexing data such as voice and video with more control. ATM
is designed for very high speeds of data transfer, such as OC-192 (10
Gbps!).

Digital Subscriber Line (DSL) and cable modem technologies are other types of
WAN connections that are being brought to the public's attention. Their
primary role so far has been to connect more people to the Internet using faster
speeds than is capable with a modem.